The present invention relates generally to interconnect structures and methods, and more particularly, to a grid array interconnect structure employing epoxy interconnects and transfer tape underfill for interconnecting high density multichip interconnect decals or land grid array packages to printed wiring boards, and interconnection methods employing the interconnect structure.
Known prior art relating to the present invention are solder attached ball grid arrays and chip scale packages. The disadvantages of these products are that they are relatively low density interconnect structures, and they are attached using solder. Furthermore, interconnection of certain semiconductor devices to printed wiring boards is hindered because of mismatches in the coefficients of thermal expansion of the printed wiring boards and the devices or carriers that hold the devices. In addition, removal of soldered devices from underlying printed wiring boards is generally difficult, costly and time consuming.
Accordingly, it is an objective of the present invention to provide for a grid array interconnect structure that may be used to interconnect high density multichip interconnect decals or land grid array packages to printed wiring boards. It is another objective of the present invention to provide for a grid array interconnect structure employing epoxy interconnects and transfer tape underfill. It is a further objective of the present invention to provide for a grid array interconnect structure that eliminates mismatch problems relating to differences in coefficients of thermal expansion between the printed wiring boards and the high density multichip interconnect decals. It is another objective of the present invention to provide for methods of interconnecting and removing high density multichip interconnect decals or land grid array packages to and from printed wiring boards.
To meet the above and other objectives, the present invention provides for a grid array interconnect structure that may be used to interconnect high density multichip interconnect decals, or packages having land grid arrays, to printed wiring boards. In accordance with the present invention, a two-sided high density multichip interconnect decal, or package having land grid arrays, is interconnected to a printed wiring board using low temperature, easy to process and reworkable, polymeric materials. In particular, the high density multichip interconnect decal or package is interconnected to the printed wiring board using a grid array interconnect structure comprised of conductive epoxy interconnects and transfer tape underfill.
Conductive epoxy, in the form of epoxy paste or pre-forms that are 0.025 inches in diameter on 0.050 inch centers is used to provide interconnection between input/output (I/O) pads of the high density multichip interconnect decal, or the land grid array of the package, and I/O pads of the printed wiring board. The nonconductive transfer tape acts as a stress absorbing underfill material, compensating for any mismatches in the coefficients of thermal expansion between the high density multichip interconnect decal and the underlying printed wiring board.
In practicing the present invention, nonconductive two-sided transfer tape is applied to the back of a two-sided high density multichip interconnect decal. Conductive epoxy is screen printed or dispensed into pre-formed holes on one side of the two-sided transfer tape and is then partially cured to a semi-rigid condition (beta staged) at a first predetermined temperature. A mylar film may be applied to the exposed surface of the transfer tape.
When the high density multichip interconnect decal is to be connected to the printed wiring board, the mylar film is removed from the surface of the transfer tape and the high density multichip interconnect decal is placed into position on the printed wiring board and a slight force is applied. This assembly is then cured at a second, relatively low temperature.
The high density multichip interconnect decal may be removed and replaced without damage to the printed wiring board. The conductive polymer interconnects and nonconductive underfill softens at a third relatively low temperature, and when softened, the high density multichip interconnect decal may be removed from the printed wiring board.
Thus, the present invention also provides for a method of interconnecting a high density multichip interconnect decal or land grid array package having a plurality of interconnect pads to a printed wiring board having a plurality of I/O pads. The method comprises applying nonconductive transfer tape having a plurality of pre-formed holes therethrough to the back of the high density multichip interconnect decal or package. Conductive epoxy is disposed into the pre-formed holes of the transfer tape. The transfer tape and conductive epoxy is partially cured to a semi-rigid condition at a first predetermined temperature to form a plurality of conductive epoxy interconnects. The high density multichip interconnect decal or package is secured to the printed wiring board using the transfer tape. Finally, the assembled high density multichip interconnect decal or package, transfer tape and printed wiring board are cured at a second predetermined temperature.